The present invention relates in general to systems for harnessing and converting the energy from ocean waves to more useable energy forms such as electric power.
More particularly, the invention relates to wave powered energy extraction systems and the components thereof, in which the oscillating ocean wave motion is used to displace a volume of air to drive a wind operated turbine connected to an electrical generator. In the preferred form of the invention, the waves are directed into a specially configured air compression chamber, in the outlet of which is arranged a suitably operable wind turbine.
To this end the various aspects of the invention include: a novel wave focusing device; an air compression chamber arrangement particularly suited for use with the novel wave focusing device; and an independently novel wind turbine operable to rotate unidirectionally under periodically reversing air flow conditions of the kind contemplated above.
It will be appreciated that whilst the various aspects of the invention are described herein as forming in combination a complete energy conversion system, each of these components, and in particular the turbine, may be suited to use in other unrelated applications. Alternatively, they may each be incorporated into similar energy conversion systems when combined with new or existing alternative component devices which are not described in detail in this document.
Concerns regarding the limited resources of traditional combustible hydrocarbon fuel sources and the damaging emissions resulting from their use, has prompted considerable research into sustainable non-polluting energy sources such as waves, wind, tidal, geothermal and solar.
Whist significant technological advances have been made in the conversion of energy from some of these alternative areas such as wind and solar, the majority of wave powered generation systems proposed to date have not been physically practical and/or economically viable.
In this regard, numerous different types of wave powered generation systems have been proposed, most of which are founded on the basic principle of using the vertical motion inherent in the movement of waves to effect a corresponding displacement of a component of the generating system. However, all of the systems proposed so far have had their limitations.
For example, one such system utilises oscillating floating paddles, the motion of which is converted directly or indirectly to electrical power. However, these floating paddle systems generally have a low energy conversion efficiency and are unable to withstand adverse weather conditions. This means either that such systems are limited to coastal locations having only moderate and predictable wave patterns, or that the systems must be removed to a suitable shelter when storms are expected.
Other systems include those based on the concept of channelling the waves through water displacement pumps, or alternatively into large accumulators or reservoirs, the hydrostatic pressure of the stored water subsequently being used to drive a turbine generator or the like. Again, the overall energy conversion efficiency is relatively low given the associated capital costs.
One of the most promising alternative types of systems proposed so far, on which the present invention is based, are those in which the vertical movement of the waves is translated to rotary movement to directly or indirectly drive a generator. In these systems the rising and falling sea water is channelled toward and harnessed within an air compression chamber. The chamber has at its exit an outlet duct or venturi, in which is located a wind turbine of a kind operable to rotate unidirectionally under the periodically oscillating air flows induced by the wave motion.
Again, the main deficiencies with these latter wave driven air turbine systems, is the restricted overall achievable energy efficiencies. This is due primarily to the limitations firstly in the means of focusing the wave energy to maximise the wave displacement amplitude, and secondly in the operating efficiencies inherent in the turbine design.
In the first case, most of the prior art wave focusing devices have relied on planar reflection of the wave front and/or channelling of the wave front through a narrowed opening such that the vertical displacement or amplitude of the wave is magnified. Others include various means to alter the formation of the sea bed to controllably disrupt the wave propagation, so as to thereby maximise the wave amplitude at a predetermined location. Once again these types of systems have been limited so far in respect of the maximum achievable wave amplification for a given level of capital expenditure.
In the second case most prior art turbines are designed for constant velocity rotation in response to fluid flow in one direction only, and as such are unable to operate continuously in response to the reversing fluid flow conditions present in wave powered applications of the kind discussed above. However, a number of specially configured unidirectional turbines have been designed for these reversing flow conditions, the most commonly used devices being based on what is known as the xe2x80x9cWellsxe2x80x9d turbine.
The original Wells turbine was of a monoplane axial fan type structure having radially extending blades of an aerofoil section that are generally symmetrical about the chord line, where the blades are fixed with their planes of zero lift normal to the axis of the rotor.
However, these early turbines were known to suffer from stalling, often resulting in the shut down of the wave energy harnessing plant. This stalling occurs due to the fact that such a turbine needs to be designed around anticipated levels of air flow, whereas the size of the waves entering the turbine chamber cannot be controlled for all occasions. Therefore, when a larger sized wave enters the chamber, its momentum causes a correspondingly greater air flow rate through the turbine blades. As the rate of rotation of the blades is unable, with its blade configuration, to increase correspondingly to counter this increased airflow, the angle of attack of the airflow to the blades increases beyond the stalling angle and the turbine shuts down.
Some later prior art devices have attempted to overcome this problem by effectively installing two monoplane Wells turbines in series resulting in a bi-plane turbine. While this modified system solves the stalling problem, it does so at a penalty to the overall efficiency. This is because it sacrifices the first set of blades by allowing them to correspondingly stall and shut down, the second set of blades then continuing operation at a reduced pace and efficiency. This is due to the total air flow rate having now been decreased and smoothed out by the stalling and interruption of the air flow by the first turbine.
These prior art turbines also usually rely on a low revving high mass construction in order to ensure smooth continuous rotation under periodically reversing driving air flows of the kind contemplated.
It will therefore be-appreciated that most prior art turbines suited to this type of application are often quite complex in design and usually have severe limitations in relation to operating conditions and/or efficiencies.
It is an object of the present invention to provide a wave energy extracting system and/or one or more of the components thereof, which overcomes or at least ameliorates one or more of the above discussed disadvantages of the prior art, or at least offers a useful alternative thereto.
According to a first aspect of the invention there is provided a plane wave focusing and amplifying structure, said structure comprising an open sided bay bounded by a generally upright wall, the wall being configured at its inner periphery to define in plan section from the bay opening two converging arms of generally part parabolic curvature, wherein the axes of symmetry of each said paraboli from which the arms are derived are parallel and the arms are joined adjacent their converging ends to form a shared apex, said wall being oriented to admit an advancing wave front in a direction generally parallel to said axes of symmetry, so that upon reflection from the wall the wave converges to an energy harnessing region near the apex at or adjacent the focus of each of said paraboli, thereby amplifying the vertical displacement of the wave at that region.
Desirably, the converging arms of part parabolic curvature are joined at the shared apex by means of an end wall portion that also defines the rear wall portion of an air compression chamber, the front portion of the chamber preferably being defined by a front wall section that extends forward of the rear portion to circumscribe a predetermined area around the energy harnessing region, the front wall section extending only partially below the anticipated water level so that the water is able to flow below the front wall and up into the chamber.
In a preferred form the wall is configured to define in plan section at its inner periphery an end part of a single parabola or close approximation thereto, wherein upon reflection from the wall the waves converge in a region at or adjacent the single focus of that parabola.
In another form, that may be less costly to construct, the structure comprises an air compression chamber wherein the rear wall portion maybe formed in part by the existing coast line and the bay is defined simply by two possibly relatively short arms of part parabolic curvature extending from the chamber walls. Generally, any compromise on the length of the parabolically curved arms is compensated for by extending the plan area of the air compression chamber that circumscribes the energy harnessing region.
Preferably, the bay is further bounded at its base by a generally planar sea bed that is of constant depth along a direction generally perpendicular to the axis of symmetry of the parabola. The depth and inclination (if any) of the sea bed can vary according to local strata and wave conditions, as well as the manner in which the amplified waves are to be harnessed for energy extraction. The general aim will be to optimise local conditions to maximise the wave magnification, ideally without the waves breaking prior to entering the harnessing region. For example, in one preferred form, the sea bed may slope upwardly toward the harnessing region to assist in further forcing the water upwardly at that location.
Preferably, the focal length of the parabola should be less than or equal to {fraction (1/7)} of a wave length of the incoming waves, which in a majority of cases results in a focal length of between 5 and 15 meters.
According to a second aspect of the invention there is provided a turbine operable to rotate unidirectionally when subjected to reversing generally axial fluid flows therethrough, said turbine included a rotor comprising:
a central hub;
a plurality of straight radially extending aerofoil sectioned blades each connected with said hub;
the cross section of each of said blades being approximately symmetrical about a line defining the maximum camber height and generally constant along its radially extending length;
whereby the approximately symmetrical shape of the blades and their orientation in relation to the hub facilitates unidirectional rotation of the rotor in response to reversing axial fluid flows therethrough.
Preferably the blades are each connected with the hub such that the included angle between the chordal plane of said aerofoil section and the axis of the hub is between 0xc2x0 and 90xc2x0 and more preferably between 0xc2x0 and, say, 45xc2x0.
Desirably, the above discussed maximum included angle is adjustable and further can preferably be reversed in synchronisation with the reversing fluid flow, to thereby optimise the angle of attack for the fluid flow in both directions.
It will be appreciated that reversing of the blade pitch can be achieved in numerous ways including, for example, the use of a motor driven bevel gear assembly disposed to rotate a central spigot on which each blade is mounted. In another variation, each blade is mounted on a spigot having an offset operating arm which cooperates with a helically splined actuating shaft which is reciprocally movable along the axis of the rotor.
In one preferred form suited to a particular set of conditions, the maximum included angle is between +30 and xe2x88x9230 and is reversible to correspond with the reversing fluid flow. In another preferred form, particularly suited for applications of the kind described herein, in which the working fluid is a gas such as air, the reversal of the blade pitching is by means responsive to a pressure transducer disposed to detect the point of reversal of the gas flow.
Desirably, the blades are equi spaced about the central hub. In some preferred forms suited to particular applications, the rotor has between 4 and 16 blades. The solidity can be highly variable often falling in the range of between 0.2 and 0.8. The preferred blade chord ratio is 18%, and the preferred blade profile comprises two merged front halves of a standard NACA 65-418 aerofoil.
According to a third aspect of the invention there is provided an ocean wave energy extracting system, said system including:
wave focusing means to magnify the periodic vertical peak to trough displacement of incoming waves at a predetermined plan location defining an energy harnessing region;
an air compression chamber having a generally submerged water inlet disposed at or closely adjacent said harnessing region to admit the periodically oscillating waves so as to displace a volume of air thereabove to thereby generate a correspondingly periodic reversing air flow;
said compression chamber also having an air outlet in which is located an air driven turbine operable to rotate unidirectionally in response to said reversing air flow.
Desirably, the turbine is one configured in accordance with the second aspect of the invention.
Preferably, the wave focusing means comprises a generally parabolic plane wave focusing and amplifying structure in accordance with the first aspect of the invention wherein the focus of the parabola falls within the predetermined plan location.
Desirably, the air compression chamber is configured to converge from the water inlet toward the air outlet so as to accelerate the air flow. In one preferred form, the chamber includes a venturi adjacent its outlet in the throat of which is disposed the air driven turbine.
In other preferred forms, the air compression chamber outlet and/or the shrouding and/or stators associated with the turbine, may include guide vanes to optimise the direction of air flow into and/or out of the turbine.